Nonspecific immunodepression and protective immunity in mice infected with Leishmania mexicana.

C57BL/6 mice infected with Leishmania mexicana showed depression of the in vitro immunoglobulin M-plaque-forming cell response to sheep erythrocytes. Immunodepression was present 3 weeks after inoculation and was maximal at 11 weeks. Thereafter, there was a gradual return to normal immunoresponsiveness correlated with the resolution of lesions. At the time of maximal immunodepression, spleen cells from infected mice diminished the plaque-forming cell response to sheep erythrocytes of normal spleen cells. On the other hand, specific responses, as exemplified by protective immunity to a challenge infection and delayed hypersensitivity responses to parasite antigens, were apparently unaffected. These responses were both present in mice bearing primary lesions and were maximal in recovered mice. The significance of these findings is discussed in relation to a current hypothesis on parasite-induced immunodepression.